Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
1999-04-26
2002-02-05
Corrielus, Jean (Department: 2631)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C375S229000
Reexamination Certificate
active
06345076
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a receiver for a digital transmission system with an incoherent transmission method, which receiver includes an equalizer for forming estimates for a sequence of symbols a[k] transmitted by a transmission channel from received symbols r[k] by means of an impulse response h[k] that describes the transmission properties.
The invention further relates to an equalizer for a digital transmission system with an incoherent transmission method and to a mobile radiotelephone for a digital transmission system with an incoherent transmission method.
BACKGROUND OF THE INVENTION
Such receivers are used in digital transmission systems, for example, in the digital mobile radiotelephone according to various international standards, which transmission systems comprise at least a transmitter, a transmission channel and a receiver. A data source in the transmitter (for example, a microphone with an A/D converter in a mobile radiotelephone) generates a sequence of binary symbols d[i]∈{0;1} which are subsequently modulated by means of an MDPSK (M-ary Differential Phase Shift Keying) modulation method. In a QPSK modulation (Quarternary Phase Shift Keying, with M=4), for example, two successive bits (00,01,10,11) in a mapper are shown on a QPSK symbol a[k]. These symbols are differentially coded in a precoder, so that symbols b[k]=a[k]•b[k−1] evolve. As a result, the symbols are not determined by the absolute phase position of the carrier frequency, but by the difference from the phase position of the previous symbol, which can be used in a receiver having an incoherent receiving method. The determination of an absolute phase position leads to problems during the demodulation, which problems are caused by phase ambiguities. With a quarternary DPSK modulation, there are relative phase differences between successive symbols b[k] of 0°, 90°, 180° and −90° (45°, 135°, −135 and −45° respectively, with &pgr;/4 QDPSK) in dependence on the symbols 00,01,10 and 11. When a differential precoding of the symbols (QDPSK) is used, this is also known as an incoherent transmission method.
The sequence of symbols b[k] is transmitted by a possibly time-variant transmission channel which has distortion and noise. In a receiver input stage the received symbol r(t) is sampled with a symbol clock T, the sampling instant kT+t
0
being determined by a synchronizer. The discrete sequence r′[k]=r(kT+t
0
is obtained then. A subsequent standardization with the average efficiency of the received symbols r′[k] leads to the symbols r[k] which have the average efficiency 1. The symbols r[k] may be described with a desired symbol y[k] to which an interference portion n[k] is added. This noise sequence n[k] may be assumed to be white Gaussian noise.
By means of an equalizer, a receiver estimates the sequence of symbols â[k−k
max
] from the sampled values of the received signal, while this sequence must be a maximum match for the transmitted sequence a[k] except for the delay k
max
. Estimates for the data sequence d[i] can be determined from the symbols â[k−k
max
] by means of a conversion of the mapping. The description of the formation of the transmission pulse, high-frequency modulation and transmission gain and, at the receiving end, the high-frequency demodulation and receiving filtering is omitted for clarity and only the baseband model is represented. The transmission properties of the whole transmission channel between the transmitter-end symbols b[k] and the received symbols r[k] are combined, in a time-invariant channel, to an overall impulse response h(t) or h[k] respectively, in the symbol clock model. In the case of a time-variant channel, that is to say, when the properties depend on time, the transmission properties of the channel are described by the channel impulse response h(&tgr;,t). In the following, this dependence on time will not be taken into account to clarify the representation. In the channel impulse response h(t) are included as transmission properties also the Inter-Symbol Interference (ISI) of the linearly distorting transmission channel, which ISI is caused by multipath propagation of the signal. The mixing of the baseband signal with a high-frequency carrier signal in non-synchronized Local Oscillators (LO) leads to a phase and frequency offset which produces additional intersymbol interference upon reception.
In an incoherent receiver, the absolute phase position of a received symbol is not determined within the symbol interval. Only the relative phase difference of successive symbols is determined. This is habitually achieved by differentiating the sampling frequency of the received signals by means of a multiplication by the conjugate-complex symbol sampling frequency shifted by one symbol interval. The absolute phase position of the carrier frequency is then eliminated from the sequence of desired signals. Also Rayleigh fading occurring in mobile radiotelephone systems causes a frequency offset of the received signal to occur, as a result of which an incoherent receiving method is advantageous.
When receivers with incoherent receiving methods have transmission channels in which a symbol received in interval k is also influenced by L−1 previous symbols, they have high bit error rates when the received symbols are detected. L denotes the number of symbols superimposed in the interval k as a result of multipath propagation, for example, which may be described with a memory length L−1 of the transmission channel (having a discrete channel impulse response of h=[h(0),h(1), . . . h(L−1)]) and leads to intersymbol interference (ISI). The superpositioning leads to a sequence of desired symbols y[k] which are described by the sum
y
[
k
]
=
∑
l
=
0
L
-
1
h
[
l
]
·
b
[
k
-
l
]
In “Digital Communications”, 3
rd
Edition, MgGraw-Hill International Editions, 1995, by John G. Proakis, is described a receiving method for differential. PSK (DPSK) with channels that have no pulse interference. From page 274 onwards is shown the reception of differentially coded, phase-modulated signals. As appears from the processing shown of the received symbol r(t), the phase position of the carrier signal need not be estimated. With the multiplication of the sample value r[k] of a received signal r(t) by the conjugate-complex value of the previous value r*[k−1], the phase position of the carrier signal disappears from the defining equation, so that only the difference between the phase angle of the signal at instant k and the phase angle of the previous signal (k−1) needs to be detected. Consequently, this MDPSK method is also referred to as an incoherent receiving method. Since the channel memory is discarded for this method, the bit error rate of channels having intersymbol interference is very high.
From the article “Nonlinear Equalization of Multipath Fading Channels with Noncoherent Demodulation”, Ali Masoomzadeh and Subarayan Pasupathy, IEEE Journal on Selected Areas in Communications, vol. 14, no. 3, April 1996, pp. 512-520 is known an equalizer for MDPSK-modulated signals. For these MDPSK signals is then proposed a receiving method with a distorting transmission channel and non-linear intersymbol interference (ISI), which interference arises from the differentiation in the receiver. In the incoherent receiver a decision feedback equalization DFE is used for the detection. Owing to the non-linear distortions as a result of the differentiation, the conventional DFE cannot be used. Therefore, it is necessary to implement a modified DFE method which also takes the non-linear distortion into account in the “Digital Communications” men
Gerstacker Wolfgang
Huber Johannes
Petersen Jürgen
Schober Robert
Corrielus Jean
Halajian Dicran
U.S. Phillips Corporation
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